How do I combine filter and projection operation in Spring EL?

Using Spring Expression Language (SpEL) we can filter a collection based on some criteria. We can also create a projection of a collection by collecting only a particular property from the collection objects.

Now you know that you have two good features of SpEL that are really powerful to use when working with collection objects manipulation. But you are wondering how to combine both of these filter and projection in one expression. Can you do this in Spring EL? The answer is yes! You can combine them both in one expression. Let’s see an example below.

We are going to use the same configuration used in the previous example:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:util="http://www.springframework.org/schema/util"
       xmlns:p="http://www.springframework.org/schema/p"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util.xsd">

    <util:list id="books">
        <bean class="org.kodejava.example.spring.el.Book"
              p:title="Essential C# 4.0" p:author="Michaelis" p:pages="450"/>
        <bean class="org.kodejava.example.spring.el.Book"
              p:title="User Stories Applied" p:author="Mike Cohen" p:pages="268"/>
        <bean class="org.kodejava.example.spring.el.Book"
              p:title="Learning Android" p:author="Marco Gargenta" p:pages="245"/>
        <bean class="org.kodejava.example.spring.el.Book"
              p:title="The Ruby Programming Language"
              p:author="David Flanagan & Yukihiro Matsumoto" p:pages="250"/>
        <bean class="org.kodejava.example.spring.el.Book"
              p:title="Einstein" p:author="Walter Isaacson" p:pages="1000"/>
    </util:list>

    <bean id="library" class="org.kodejava.example.spring.el.Library">
        <property name="bookTitles" value="#{books.?[pages gt 250].![title]}"/>
    </bean>

</beans>

In the configuration above, when we define the library bean we set its bookTitles property using the filtering and projection operator. First we take only books that have more that 250 pages and then we create the projection that contains only the book title. So this expression give us all the book’s title of a book that has more than 250 pages.

To make the example complete here again the definition of the Book and the Library class.

package org.kodejava.example.spring.el;

public class Book {
    private Long id;
    private String title;
    private String author;
    private String type;
    private int pages;

    // Getters & Setters
}
package org.kodejava.example.spring.el;

import java.util.List;

public class Library {
    private List<Book> books;
    private List<String> bookTitles;

    // Getters & Setters
}

The main class the run the configuration file:

package org.kodejava.example.spring.el;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class SpELFilterProjectionExample {
    public static void main(String[] args) {
        ApplicationContext context =
            new ClassPathXmlApplicationContext("spel-filter-projection.xml");

        Library library = context.getBean("library", Library.class);

        for (String title : library.getBookTitles()) {
            System.out.println("title = " + title);
        }
    }
}

The result of the code snippet:

title = Essential C# 4.0
title = User Stories Applied
title = Einstein

How do I filter collection members based on some criteria using Spring EL?

In some of the previous example you have seen that we use the square-braces [] operator to select items from collection. In this blog post you will learn how to filter some members of a collection with a certain criteria while using the Spring EL. To do this Spring EL give you another special operator, the filter operator which can be typed like .?[]. Where you can define the filter criteria inside the braces.

Beside the filter operator .?[]> there are also operator that can select the first and the last matching items from collection. You can use the .^[] for selecting the first match and the .$[] operator to select the last match items from collection respectively.

As an example we will demonstrate here that we want to find a collection of books that have pages more than 250 and assign it to Library bean’s books properties. So let’s begin by defining a class called Book and Library which will have the following properties:

package org.kodejava.example.spring.el;

public class Book {
    private Long id;
    private String title;
    private String author;
    private String type;
    private int pages;

    // Getters & Setters

    @Override
    public String toString() {
        return "Book{" +
            "title='" + title + '\'' +
            ", author='" + author + '\'' +
            ", pages=" + pages +
            '}';
    }
}
package org.kodejava.example.spring.el;

import java.util.List;

public class Library {
    private List<Book> books;

    public List<Book> getBooks() {
        return books;
    }

    public void setBooks(List<Book> books) {
        this.books = books;
    }
}

After creating the Book and the Library class let’s now create the Spring configuration file for our demo. We will create a file and call it as spel-select-collection.xml with the following lines of configuration in it.

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:util="http://www.springframework.org/schema/util"
       xmlns:p="http://www.springframework.org/schema/p"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util.xsd">

    <util:list id="books">
        <bean class="org.kodejava.example.spring.el.Book"
              p:title="Essential C# 4.0" p:author="Michaelis" p:pages="450"/>
        <bean class="org.kodejava.example.spring.el.Book"
              p:title="User Stories Applied" p:author="Mike Cohen" p:pages="268"/>
        <bean class="org.kodejava.example.spring.el.Book"
              p:title="Learning Android" p:author="Marco Gargenta" p:pages="245"/>
        <bean class="org.kodejava.example.spring.el.Book"
              p:title="The Ruby Programming Language"
              p:author="David Flanagan & Yukihiro Matsumoto" p:pages="250"/>
        <bean class="org.kodejava.example.spring.el.Book"
              p:title="Einstein" p:author="Walter Isaacson" p:pages="1000"/>
    </util:list>

    <bean id="lib1" class="org.kodejava.example.spring.el.Library">
        <property name="books" value="#{books.?[pages gt 250]}"/>
    </bean>

    <bean id="lib2" class="org.kodejava.example.spring.el.Library">
        <property name="books" value="#{books.^[pages gt 250]}"/>
    </bean>

    <bean id="lib3" class="org.kodejava.example.spring.el.Library">
        <property name="books" value="#{books.$[pages gt 250]}"/>
    </bean>

</beans>

The first thing that you can see in the configuration above is the <util-list> where we create a list of Book beans. Next we have three beans definition of the type Library where the books property was assigned with a collection of beans selected from the books list.

The lib1 will contains books that have pages more than 250. The lib2 will contains the first book found to have more than 250 pages while the lib3 will contains the last book found to have more than 250 pages.

Below is the code that will run our configuration file and the example output it will produce.

package org.kodejava.example.spring.el;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class SpELFilterCollection {
    public static void main(String[] args) {
        ApplicationContext context =
            new ClassPathXmlApplicationContext("spel-filter-collection.xml");

        System.out.println("Library 1");
        Library lib1 = (Library) context.getBean("lib1");
        for (Book book : lib1.getBooks()) {
            System.out.println("    Book = " + book);
        }

        System.out.println("Library 2");
        Library lib2 = (Library) context.getBean("lib2");
        for (Book book : lib2.getBooks()) {
            System.out.println("    Book = " + book);
        }

        System.out.println("Library 3");
        Library lib3 = (Library) context.getBean("lib3");
        for (Book book : lib3.getBooks()) {
            System.out.println("    Book = " + book);
        }
    }
}
Library 1
    Book = Book{title='Essential C# 4.0', author='Michaelis', pages=450}
    Book = Book{title='User Stories Applied', author='Mike Cohen', pages=268}
    Book = Book{title='Einstein', author='Walter Isaacson', pages=1000}
Library 2
    Book = Book{title='Essential C# 4.0', author='Michaelis', pages=450}
Library 3
    Book = Book{title='Einstein', author='Walter Isaacson', pages=1000}

How do I set a filter on a logger handler?

This example give you an example on how to set the Filter of a logger handler. In the code below we implements the Filter on the FileHandler to log only a Level.SEVERE message. Other level log will not recorded to the file.

package org.kodejava.example.util.logging;

import java.util.logging.*;
import java.io.IOException;

public class LoggerFilter {
    public static void main(String[] args) {
        Logger logger = Logger.getLogger(LoggerFilter.class.getName());

        FileHandler handler = null;
        try {
            handler = new FileHandler("myapp.log");
            handler.setFilter(new Filter() {
                public boolean isLoggable(LogRecord record) {
                    // When the LogRecord level is equals to Level.SEVERE the 
                    // message is recorded to the file.
                    return record.getLevel().equals(Level.SEVERE);
                }
            });
        } catch (IOException e) {
            logger.log(Level.SEVERE, "Fail to create logger handler", e);
        }

        logger.addHandler(handler);

        logger.info("Information message");
        logger.warning("Warning message");
        logger.severe("Severe message");
    }
}